Predicate matchMappingFlowStep
double star pattern: subject flows to the variable, possibly via a conversion
syntax (toplevel): case {**var}:
Dictionary content flows to the double star, but all mentioned keys in the mapping pattern should be cleared.
Import path
import semmle.python.dataflow.new.internal.MatchUnpackingpredicate matchMappingFlowStep(Node nodeFrom, Node nodeTo)